The fixed stars are traditionally just the relatively nearby stars (e.g., those that historically define the constellations) that are moving in very similar orbits to the Solar System's orbit around the Milky Way.
The fixed stars can be used to approximately measure rotation relative to the observable universe: i.e., to the bulk mass-energy of observable universe): i.e., rotation that counts as an accelerated motion relative to an inertial frame. Yours truly prefers to say rotation relative observable universe rather than "absolute rotation" since it is more descriptive of what is actually done to measure "absolute rotation" using International Celestial Reference System (ICRS) (see Wikipedia: International Celestial Reference System and its realizations: Realizations). Also Wikipedia absolute rotation is NOT consistent with rotation relative to the observable universe.
The fixed stars can be used to approximately measure rotation relative observable universe because their peculiar velocities on average give a rather slow rotation relative to the observable universe.
Historically, the fixed stars were always used until well into 20th century for measuring "absolute rotation" at first because this was thought to be exactly correct and later because it was the best that could be done. The fixed stars are still used for measuring rotation relative to the observable universe when you do NOT need the highest accuracy/precision.
Yours truly has almost broken the habit saying "relative to the fixed stars" when yours truly means "relative to the observable universe" which is the exactly correct thing to say when you are discussing "absolute rotation".